Many scientists believe that petroleum comes from the remains of millions of long-dead plants and animals. One theory holds that most petroleum comes from the decomposition of very tiny marine organisms and plants, although a number of larger animals may have contributed, as well. The tiny creatures that compose most oil are known as diatoms. Although they were not technically plants, diatoms did convert sunlight into energy just as plants do. Diatoms live in the top few meters of large bodies of water, such as lakes and oceans, and serve as a source of food for a wide range of marine animals.
To create oil, the conditions may have included a thick layer of diatoms and other organic remains piled up on the floors of the sea and lakes, gradually getting buried in sediment and compressed. Extreme heat and pressure combined to cause a number of chemical reactions that convert the organic matter first into source rock called shale and then into oil and natural gas. Oil starts out stored under ground in reservoir rock. This includes a wide range of rock varieties, including limestone, dolostone, and sandstone. These kinds of rock are porous and have space inside them where oil can be trapped and stored, just as a sponge can trap water or other liquids.
Oil held in reservoir rock then moves around as layers of earth shift. For energy companies to access oil for exploration and extraction, the substance must build up in reservoirs composed of less porous trap or seal rocks that block oil from passing through. The reservoirs formed by trap rock cause oil to pool in some areas, where prospectors and extraction companies can access and remove the oil.
About Sentry Energy: Based in Addison, Texas, Sentry Energy conducts exploration for and extraction of oil and gas. The firm enables partners to diversity their business interests into the energy sector.